A gas turbine engine includes a compressor section, a combustion section, and a turbine section. Disposed within the turbine section are alternating rows of rotatable blades and static vanes. As hot combustion gases pass through the turbine section, the blades are rotatably driven, turning a shaft and thereby providing shaft work for driving the compressor section and other auxiliary systems. The higher the gas temperature, the more work that can be extracted in the turbine section and the greater the overall efficiency. In order to increase the turbine section operating temperature capability, cobalt and nickel based superalloy materials are used to produce the turbine airfoil blades and vanes. Such materials maintain mechanical strength at high temperatures.
The static vanes, disposed between the rows of rotating blades, stabilize and direct the gas flow from one row of rotating turbine blades to the next row, with a nozzle area defined by the spacing between the adjacent vanes. Such gas flow stabilization optimizes the amount of work extracted in the turbine section. Generally, the nozzle flow area is assigned a series of classification numbers that correlate to the volumetric gas flow. This allows comparison of flow properties between vanes of complex geometry. The nozzle area is therefore defined for convenience in terms of a class size.
In service, deterioration of the vane surface(s) occurs due to oxidation and metal erosion caused by abrasives and corrosives in the flowing gas stream impinging on the vane. In addition, high gas loadings at high temperature promote distortion of the vanes, thereby enlarging the nozzle flow area, with a consequent loss in turbine efficiency. During a periodic engine overhaul, the vanes are inspected for physical damage and evaluated to determine the degree of flow area change (typically by determining the effect on nozzle classification). Before such vanes can be returned to the engine, any eroded material must be replaced and the vanes returned to the original classification (referred to as being “reclassified”). In addition, any vanes which suffer a loss of metal or a change in shape due to coating removal or repair must be reclassified.
Several methods exist for reclassifying. One method involves hot striking or otherwise bending the trailing edge of the vane, narrowing the gap between adjacent vanes. However, such bending introduces stresses that may produce cracks in the vane. Such bending may also cause excessive distortion of the vane, preventing the proper fit and seal of the internal cooling tubes. The fixturing devices, which hold the vanes during bending, may also distort the vane platform or crush the vane pedestal. Even if bending stresses can be reduced, hot forming or bending of certain high temperature alloys is not performed to avoid affecting material properties such as fatigue strength. Because the bending process does not add metal to the vane surface, there is no strength contribution. As such, the repaired vane does not have as long a useful life as a new vane.
Another method for reclassifying turbine vanes involves the addition of an alloy to the deteriorated vane surface by a combined weld/plasma spray process, such as that described in U.S. Pat. No. 4,028,787 to Cretella et al. This process requires the addition of weld beads to the worn surface for reinforcement, with a number of plasma sprayed layers of the alloy then added to achieve the proper alloy thickness. This procedure is very labor intensive requiring a welder to add a number of weld beads to a small surface, clean the vane, and then add a number of plasma spray layers. In addition, the vane may be damaged due to the thermal stresses involved in the welding operation.
Another issue with the weld/plasma spray process involves the specific area of deterioration. It is to be expected that deterioration will be more severe at the narrowest nozzle dimension where the velocity of the gas flow is highest. During the plasma spray process, alloy is added to the surface in very thin layers, forming a broad even pattern. After completion of the plasma spray, the excess material must be removed from non-eroded areas of the vane. If the deterioration is severe in specific areas, numerous layers of the alloy must be added and much of it removed from the non-eroded areas. Such a procedure is time consuming and wasteful of the alloy materials involved.
Still another method for refurbishing gas turbine vanes is shown in U.S. Pat. No. 4,726,101 to Draghi et al. In this method, a build up of alloy in the wear area is accomplished by controllably applying layers of a tape of uniform thickness to the vane. The tape includes a mixture of a binder and an alloy powder, which is compatible with the substrate alloy, with the mixture formed into a sheet of uniform thickness and having an adhesive backing. After applying the tape in layers to a desired thickness, the vane is heated to a temperature at which the binder and adhesive decompose and the alloy in the tape diffusion bonds with the substrate alloy.
Yet another method for refurbishing a gas turbine vane covered by a protective coating is shown in U.S. Pat. No. 5,142,778 to Smolinski et al. In this method, the protective coating is first removed from the surface of the vane. Thereafter, material is added to surfaces of the vane in the areas requiring repair or replacement of the eroded material and bonded to the surfaces. A laser beam is then directed at the surface in the distorted areas such that localized areas of the surface of the distorted areas are melted, solidified, and cooled to ambient temperature to form a recast layer. Any excess material is removed from the surface and the protective coating is reapplied.
Another method for refurbishing gas turbine vanes is disclosed in U.S. Pat. No. 5,522,134 to Rowe et al. The method of restoration in Rowe et al. includes the steps of cutting a plate of pre-sintered material that is either cobalt or nickel-based and machining the plate so the plate includes the appropriate thicknesses throughout. Thereafter, any protective coatings are removed from the material and the airfoil to which the material will be added is degreased and cleaned. The plate is then positioned over the area of the airfoil that needs to be repaired or refurbished and the plate is welded to the airfoil by resistance tack welding.
The current industry practice is to use a single layer preform for restoring dimensions of an airfoil, wherein the single layer includes two intermixed or blended components, namely a base alloy and a low melt phase alloy. The airfoil and the preform are subjected to braze melt and diffusion heat cycles. Due to certain alloying elements in the low melt phase alloy, the single layer preform has a lower oxidation resistance than preferred, thus possibly leading to premature removal of turbine engines or airfoils.